1. Field of the Invention
The present invention relates to a position control method, a program and a storage medium, a position control device, and an optical disk device, and more particularly, to a position control method for controlling a position of an object lens in a direction perpendicular to a tangential direction of a spiral or concentric track formed on a recording surface of a recording medium, a program and a storage medium used in an optical disk device, a position control device for controlling a position of an object lens in a direction perpendicular to a tangential direction of a spiral or concentric track formed on a recording surface of a recording medium, and an optical disk device including the position control device.
2. Description of the Related Art
Along with improvements in the capability of personal computers, it has become possible to process audio-visual (AV) data. Because the volume of AV data can be very large, optical disks having large capacities, such as CDs (Compact Disk) and DVDs (Digital Versatile Disk), are attracting more attention. As the price of the optical disks continues to go down, optical disk devices for accessing optical disks, which are used as common computer peripherals, are widely spread.
An optical disk device forms a light spot on a recording surface of a recording medium, for example, an optical disk, on which spiral or concentric tracks are formed, and thereby records data on or deletes data from the optical disk, or reproduces data based on light reflected from the recording surface. On an optical disk, there are mark regions (pit) and space regions, which have different reflectivity, and the data on the optical disk are represented by lengths and combinations of the two regions. The optical disk device has an optical pickup for emitting a light beam and receiving reflected light from the recording surface.
Generally, an optical pickup has an object lens, an optical system for directing a light beam from a light source to the recording surface and guiding the reflected light beam (returning light beam) from the recording surface to a specified position for light reception, and a light reception element located at the light reception position. The light reception element outputs signals which include not only information for reproducing the data recorded on the recording surface, but also information for controlling the positions of the optical pickup itself and the object lens (the latter information is referred to as “servo information”).
In order to correctly record data at specified positions on the recording surface, or correctly reproduce data recorded at specified positions on the recording surface, the light spot must be precisely formed at the target position on the recording surface. For this purpose, it is necessary to accurately detect the position where the light spot is formed. Various methods have been proposed to accurately detect the position of a light spot. For example, the three-spot method is a well known method for detecting the position of a light spot on a recording surface.
In the three-spot method, the light beam emitted from the light source is divided into a main beam and two sub-beams.
FIG. 1 is a diagram for schematically explaining a position relation of light spots in the three-spot method.
As shown in FIG. 1, marks are formed on the recording surface along concentric or spiral tracks (only sections of tracks are illustrated in FIG. 1). In FIG. 1, the symbol Dtr indicates the direction perpendicular to a tangential direction of a track (hereinafter, Dtr is referred to as “a tracking direction”).
The main beam and two sub-beams form three light spots on the recording surface, specifically, a light spot SP1 of the main beam and two light spots SP2 and SP3 of the two sub-beams. As shown in FIG. 1, the light spots SP1, SP2, and SP3 are not formed along the tangential direction of a track; the light spots SP2 and SP3 shift in the tracking direction Dtr, specifically, by ¼ of a track pitch (denoted by Tp/4), respectively.
The returning light beams of the two sub-beams are received by two light reception elements, respectively, and a tracking error signal is obtained from the difference between the amounts of light detected by the two light reception elements.
If the object lens is moved along the tracking direction Dtr, the positions of, the three light spots also move along the tracking direction Dtr.
FIG. 2 is a diagram for schematically explaining an on-track determination position in the tracking error (TE) signal obtained by the three-spot method.
As shown in FIG. 2, the tracking error signal TE is a periodic signal. When the level of the tracking error signal TE is or becomes zero (that is, at the zero-crossing point), then a so-called “on-track condition” occurs. In other words, when the level of the tracking error signal TE is or becomes zero, the light spot SP1 of the main beam is substantially at the center of a track (hereinafter, referred to as “track position”.)
The level of the tracking error signal TE changes when the position of the object lens changes. For example, when the object lens is moved in the direction A (as shown in FIG. 2), the level of the tracking error signal TE increases gradually; when the level of the tracking error signal TE becomes zero (the zero-crossing point), it indicates that the on-track condition occurs, in other words, the light spot SP1 of the main beam is substantially at the center of a track. On the other hand, if the object lens is moved in the direction B, the level of the tracking error signal TE decreases gradually; and when the level of the tracking error signal TE becomes zero (the zero-crossing point), it indicates that the on track condition occurs, and the light spot SP1 of the main beam is at the track position. In this way, the position of the object lens can be controlled.
Exploiting this phenomenon, servo control (tracking control) is performed to maintain the light spot of the main beam to be on the track position during recording or reproduction operations.
FIG. 3 shows an example of waveforms of the tracking error signal, a tracking cross (TC) signal and a RF signal observed on an oscilloscope. The RF signal includes information of reproduced data.
In a recording medium on which guide grooves are not formed, such as a CD-ROM, the difference between the reflectivity of the mark regions and the reflectivity of the space regions largely contributes to the tracking error signal. On the other hand, in an optical disk on which guide grooves are formed, diffraction of the incident light beam near edges of the grooves largely contributes to the tracking error signal.
For example, Japanese Laid Open Patent Application No. 3-142714, Japanese Laid Open Patent Application No. 3-142722, Japanese Laid Open Patent Application No. 5-6551, and Japanese Laid Open Patent Application No. 7-57279 disclose background art of this technical field.
Along with a rapid increase in the number of users, recording media are being fabricated and supplied by more and more manufacturers. Among the large amount of recording media, there appear products having low quality, for example, having very small differences between the reflectivity of the mark regions and the reflectivity of the space regions, and also products not meeting the standards of the recording media.
Specifically, it is known that the difference between the reflectivity of the mark regions and the reflectivity of the space regions largely influences the level of the tracking error signal in a recording medium having no guide groove, but the above low-quality recording media cannot generate a tracking error signals having a sufficiently high level, and thus, it is difficult to precisely set a light spot on a target position. In addition, on the low-quality recording media, because the reflectivity is not uniform even on the same recording surface, it is difficult to perform stable position control. Furthermore, the RF signal associated with the nonstandard recording media may be abnormal.
Meanwhile, because these low-quality recording media or nonstandard recording media have been in the market in large amounts, they are being used by many users, and some important data have been recorded on such kinds of recording media. In this situation, the users are desirous of an optical disk device that can normally access a recording medium even when the recording medium is of low quality or is not a standard one.